If you open a Word document and can view it but can't make edits to it, the document is in Protected View mode. The document's creator has enabled this mode to protect the content of the document. If you downloaded the document from the Internet, Word opens it in Protected View mode. You can re-enable editing on the document.
Protected View mode prevents the original document from being modified. If you do make edits to a document with Protected View, you have to save the file with a new name or in another location, but your changes aren't saved to the original document. When a document is downloaded from the Internet, this serves to protect you from any malicious content the file might contain.
If the document is not password-protected, you can disable Protected View mode. Click the "Review" tab and click "Restrict Editing." All restrictions are removed, and you can edit the document. If the document is password-protected, you need to contact the document's creator for the password.
If you downloaded the document from the Internet, you see a message bar that says, "This file originated from an Internet location and might be unsafe. Click for more details." Since files on the Internet could have viruses or malicious macros in them, Word opens them in Protected View. If you trust the file, click "Enable Editing" on the message bar to edit the document.
Accessing Documents From Office 365
Office 365 users can invite you to collaborate on a document. They can send you a guest link or have you sign in with your Microsoft account to access the document. If you can only open the document in a read-only mode, your permissions to the document need to be changed. Ask to be added to the Members group, which will give you permissions to edit documents, or ask for a new guest link that has Edit permissions.